1. Field of the Invention
The invention is a mechanochemical polishing abrasive for use in the polishing of hard substrates such as ceramics, crystalline-materials, glass and similar materials which require highly polished surfaces.
The economics of polishing and machining ceramics can often be the most costly part of the ceramic production process. Economic considerations for polishing ceramics involve both the time and the consumable products employed. For final polishing of ceramics, diamond abrasives are extensively used, the diamond abrasives are expensive and the polishing process using diamond abrasives is slow.
Conversely, the machining of ceramics can be very damaging. Unlike the machining of metals which are ductile, ceramics are generally very brittle. The brittle nature of ceramics makes them very sensitive to subsurface fracturing. This subsurface damage adversely affects significant physical properties of the ceramics. Such physical properties adversely affected by machining include a reduction in the ceramics strength, changes in the ceramic magnetic properties, and even changes in the electronic properties of the ceramics.
The machining of advanced ceramics has traditionally been accomplished by hard abrasives such as diamond or silicon carbide. Though this has produced surfaces that are acceptable under certain circumstances, there still remains a certain degree of surface and subsurface damage utilizing these compounds. The use of softer polishing abrasives, such as colloidal silica, for machining advanced ceramics has been examined. Colloidal silica has been shown to polish alumina, silica, and silicon.
In fact, colloidal silica has been used extensively to polish silicon chips. However, the use of colloidal silica to polish advanced ceramics tends to produce a polished advanced ceramic product with substantial phase relief. The relief is believed to be caused by the chemical dissolution of selective grains on the advanced ceramics by the colloidal silica.
2. To Prior Art
The use of colloidal silica to polished silicon surfaces, metals, glass, garnets and sapphires is disclosed in the article H. W. Gutsche and J. W. Moody, "Polishing of Sapphire with Colloidal Silica", J. Electrical Chemical Soc. 125, No. 1, Pages 136-138, (1978). The article discloses that colloidal silica has a chemical effect on the harder sapphire material allowing the colloidal silica to polish the sapphire. The article is silent concerning combining colloidal silica with other polishing substances to polish hardened materials.
The use of CaCO.sub.3, BaCO.sub.3 and MgO as mechanochemical polishing abrasives is disclosed in a report by H. Bora and R. J. Stokes, Study of Mechanochemical Machining of Ceramics and the Effect on Thin Film Behavior, U.S. Government Report N00014-80-C-0437-1 (Apr. 30, 1981). The report details the polishing of thin layers of MgO, Al.sub.2 O.sub.3 and Si by various abrasives including rock salt, calcite, fluorite and various other abrasives including window glass. The three compounds mentioned above were discovered to be capable of mechanochemically polishing one or more of the above materials. None of the mechanochemical abrasives used were combined with colloidal silica.
The mechanochemical polishing of sapphire, silicon, and quartz crystals is disclosed by N. Yasunaga, U. Tarumi, A. Obara, "Mechanism and Application of the Mechanochemical Polishing Method Using Soft Powder" The Science of Ceramic Machining and Surface Finishing II, NBS special publication 562, U.S. Government Printing Office, Washington, D.C., pages 171-183 (1979). The sapphire, silicon and quartz were polished with wet and dry mechanochemical media. The mechanochemical media included BaCO.sub.3, Fe.sub.3 O.sub.4, CeCO.sub.2, SiO.sub.2, CeO.sub.2, diamond and MnO.sub.2. The primary focus of the article is a description of the formation of crystalline silica materials in the mixed powder abrasive during the workpiece polishing. The crystalline materials were produced by polishing the hard materials described above at high temperatures and pressures using the mixed powder. The article does not disclose the use of colloidal silica in any manner for mechanochemical polishing.